Metal insert for vacuum-tight sealing



METAL INSERT FOR VACUUM-TIGHT SEALING Filed Sept. 26, 1940 ELONGATION-CM. PER CM.

TEMPERATURE DEGREES CENTIGRADE /NVEN TOR A TTmQ/VEY Patented Feb. 13,1945 METAL msen'r FOR. VACUUM-TIGHT SEALING Walter E. Kingston,Emporium, Pa., asslgnor to Sylvania Electric Products Inc., acorporation of Massachusetts I Application September 26, 1940, SerialNo. 358,476

seal employs a glass having the following com- 1 Claim.

This invention relates to special alloy metals and more particularly tometal inserts for providing a vacuum-tight seal to. glass.

A principal object of theinvention is to provide a special alloy whichis capable of forming a satisfactory vacuum-tight seal with soft glass.

Another object is to provide a metal insert for vacuum-tight sealing tosoft grades of glass, and the metal is of a special alloy which isrelatively cheap to manufacture and is capable of easy working such asrolling, swaging, bending and the like, the metal being directly sealedto the glass so as to form therewith aunitary rigid body.

Other objects and advantages not specifically enumerated will beapparent after a consideration of the following detailed descriptionsand the appended claim. 7

In the drawing which shows by way of example one typical embodiment ofthe. invention, the single figure thereof shows thetemperature-elongation characteristics of the metal insert according tothe invention. v

In certain devices, for example electron discharge tubes, it isnecessary .to provide a glass-tometal seal which will remainvacuum-tight over long periods and over relatively wide temperatureranges. While this is true in the case of ordinary flexible lead-inwires, it is especially true where the metal part to be sealed into theglass is olf rigid construction. Thus .there is disclosed in applicationSerial No. 145,198, filed May 28, 1937 (issued Patent No. 2,238,025), aradio tube header consisting of a glass button-like member into whichare sealed the various contact prongs in the form of rigid metal rods.While devices of this category have been successfully manufactured in1arge quantities, the cost of manufacture has been increased over theordinary radio tube because of the cost of the usual metal alloy thathav been deemed necessary to effectuate the vacuum-tight seal. On theother hand, it is possible to seal a slightly harder glass together witha chromium-iron alloy, e, g., Allegheny 55. One type of alloy that hasbeen used in this kind of radio tube header consists of iron, nickel,cobalt and chromium and is quite costly to manufacture and work.

I have found that it is possible to make devices such as disclosed insaid U. S. Patent No. 2,238.025, with soft glass headers and by usingfor the prongs an alloy which is relatively cheap to manufacture andwork. Whil the invention is not limited to any particular kind of soft;glass, it is particularly useful where the glass-to-metal position:

S10: PbO A1203 CaO NazO K20 MmOa 63.1% 20.2% 0.28% 0.94% 7.6% 5.5% 0.88%

After extended investigation, I have also found that in order to providean alloy which is satisfactory for use in such devices as radio tubeswherein the vacuum is of the order of 10- mm. of mercury or less, thefollowing conditions must be fulfilled.

.1. The thermal expansion curves of the alloy and glass should matchover the temperature range of the glass up to softening point thereofand the metal is Wetted by the glass, that is, the metal oxide dissolvespartly in the glass.

2. The oxide layer on the metal mustadhere tenaciously to the metalcore.

3. A protective oxide layer must be formed on the metal core bypre-oxidation, so as to prevent the formation of a flaky oxide duringsubsequent heating of the alloy, for example the heat encountered insealing the alloy into glass.

While the first of the above conditions has lon been recognized in thevacuum-tube art, the importance of the second and third conditions havebeen ignored with the result that it has not been found possible to useordinary ternary alloys of iron, nickel and chromium to form avacuumtight seal to soft glasses.

The tenaciou adherence of the'oxide to the metal as set forth in thesecond of the above conditions and which can be obtained by the methodsdisclosed hereinbelow, makes it possible to eliminate all the specialprecautions of annealing the glass to metal seal which are necessary inknown seals of hard glass and known alloys. In these prior sealselaborate annealing procedure must be resorted to in order to avoid aseparation between the metal and the oxide after the seal has been made.Otherwise practically speaking it is impossible to cool the metal andglass in a seal at the same rate, unless very elaborate annealingprocesses are used, due to the greater conduction of heat away from themetal part. This difi'erential in the cooling temperatures, and theassociated differences in expansivity put a great strain on the oxide tometal bond. Unless this bond is extremely tenacious there will be atendency to break away at this part. With the great strength andadherenceof the oxide, formed according to this invention, to the metal,seals can be cooled in production equipment without annealing iffiecessary and still obtain vacuum-tight condions.

In accordance with the invention, the alloy consists of approximately 42percent nickel; 4

to 8 percent chromium, 0.1 to 2 percent of aluminum, and the balancesubstantially entirely of iron and a small percentage of manganese ofthe order of 0.25 percent to 0.4 percent. I have found that such analloy in addition to having desirable expansion coefficients over a widepounds per square inch. This is particularly desirable where the alloyforms the rigid con-. tact prongs of a radio tube. I have found thatwhen the metal parts which are to be sealed in a vacuum-tight manner tocorresponding matched glasses are of known alloys, e. g., those soldrespectively under the trade names Fernico, Fernichrome, Kovar'.andAllegheny 55, the addition of a small percentage of aluminum will alsoproduce the required strong bond between these known alloys and theiroverlying metal oxide layers. The addition of a small percentage of thesaid metal to said known alloys also enables the seal with theircorresponding matching glasses to bemade with considerably betterresults, and without requiring special annealing precautions which areusually required between said known alloys and their matching glasses.For a detailed description of said known alloys reference may be had toU. S. Patents 1,942,260 and 2,071,196. The composition of the alloysFernico and Kovar is the same and consists of approximately'54% iron;28% nickel; and 18% cobalt. Ferni chrome consists of approximately 37%iron; 30% nickel; 25% cobalt and 8% chromium. Allegheny #55 consists ofapproximately 0.35 carbon; 1.0% manganese; 0.6% silicon (maximum);23-30% chromium; 0.6% nickel and the balance iron.

If desired, the alloy can be annealed at a temperature of 1000 C. so asto render it duc- *tile. Furthermore, by pre-oxidiaing the alloy at atemperature of about 1300 C. in a suitable atmosphere, for example anatmosphere of moist Hz, a tightly adherent chromium oxide layer isformed on the exterior surface, which oxide facilitates wetting by themolten glass and the formation of a, vacuum-tight bond or weld uponsubsequent cooling. I have found that by using the range of proportionsof alloy constituents as mentioned above, it is possible to vary them toprovide a corresponding series of alloys whose mean linear expansionbetween zero and 300 centigrade can be given any desired value bedottedcurve of Fig. 1 represents the expansion characteristic Of'a, softglass, such for example as that described hereinabove, which has asoftening point at approximately 475: C.

Beyond this softening point the glass begins,

to sag as indicated by the section la 01' the curve. However, byextrapolation, the equiv alent elongation can be calculated and isrepresented by the section 4b -of the curve.

Curve No. 1 represents the expansion characteristics upon heating andcooling of an alloy of the above mentioned composition containingapproximately 42 percent nickel; 4 to 8 percent chromium, the balancebeing iron with a small percentage of manganese. It will be seen thatthis alloy matches quite closely the characteristics of the glass. Ihave found that the match is sumciently close to enable the manufactureof satisfactory radio tube headers of the type disclosed in said U. S.Patent No. 2,238,025. which are free from undesirable strain around theprongs, up to the softening point of the glass, with the result that thevacuum-tight character of the seal remains permanent.

Curve No. 2 shows the expansion characteristic of an alloy consisting of42 percent nickel; 8 to 12 percent chromium, and the balance iron.Likewise curve No. 3 illustrates the expansion characteristics of analloy consisting of 42 percent nickel and the balance iron The totalimpurities including the manganese should not appreciably exceed 0.885percent with the carbon impurities not appreciably greater than 0.15.percent. Thus in one alloy that was found to have the desired propertiesfor vacuum-tight sealing, the alloy in addition to containing the iron,chromium, nickel, aluminum, manganese constituents as described above,contained the following impurities having negligible eifect on thedesirable properties of the alloy.

Percent Carbon 0.15 Silicon 0.30 Sulphur 0.020 Phosphorus 0.015

An increase of the carbon contents will increase the hardness of thealloy, and it is'thus possible to control the hardness of the materialby addition of carbon up to 0.55% to any desired degree within limits.

This application is a continuation-in-part of application Serial No.336,515, filed April 25,

What I claim is:

A metal insert for use in sealed highly evacuated devices such aselectron tubes and the like having a soft glass portion to which theinsert is to be sealed, said insert consisting of a core

